In the design of silver halide light-sensitive multilayer photographic materials, it is desirable to minimize the dry thickness of the coated layers. Layer thinning is advantageous for reasons such as improved image sharpness due to reduced light scattering during exposure and increased developability due to shorter diffusion paths through the multilayer structure. This increase in developability can lead to lower silver and/or coupler coated levels, hence lower materials cost.
Photographic dye-forming couplers, as well as other hydrophobic photographically useful compounds, are generally incorporated into a layer of a photographic element by first forming an aqueous dispersion of the couplers and then mixing such dispersion with the layer coating solution. An organic solvent is generally used to dissolve the coupler, and the resulting organic solution is then dispersed in an aqueous medium to form the aqueous dispersion.
The organic phase of these dispersions frequently includes high-boiling or permanent organic solvents. Permanent high boiling solvents have a boiling point sufficiently high, generally above 150.degree. C. at atmospheric pressure, such that they are not evaporated under normal dispersion making and photographic layer coating procedures. Permanent high-boiling coupler solvents are primarily used in the conventional "oil-protection" dispersion method whereby the organic solvent remains in the dispersion, and thereby is incorporated into the emulsion layer coating solution and ultimately into the photographic element.
In order to reduce the coated thickness of photographic layers, it is essential to minimize the amount of permanent coupler solvent coated in the element. In fact, reductions in coupler solvent level also afford concomitant reductions in gelatin level which leads to further reductions in coated dry thickness. U.S. Pat. No. 5,173,398, e.g., discloses photographic elements with coupler-containing layers having substantially no high-boiling solvent, wherein the couplers are incorporated in the layer in the form of precipitated dispersions. However, coupler solvent reduction can also result in excessive crystallization of the dispersed organic compounds in an aqueous dispersion or coating solution with photographic compounds that have a tendency to crystallize. These crystallization problems can cause filter-plugging during the manufacture of photographic materials or may result in physical defects in the coated product. Another difficulty with coupler solvent reduction is that the reactivity of the dispersed photographically useful chemical, such as a dye-forming coupler, may be too low to produce adequate dye density upon processing the photographic material.
Dispersions of photographic couplers made without using permanent coupler solvent are well-known in the art. Such dispersions are generally made with auxiliary solvents which are removed from the dispersion prior to coating. Auxiliary solvents may be water immiscible, volatile solvents, or solvents with limited water solubility which are not completely water miscible. In fact, there are many photographic compounds which can be dispersed with or without permanent solvent with no crystallization problems, as noted in U.S. Pat. No. 2,801,170. However, when the dispersed photographic compound is prone to crystallization, the tendency to crystallize generally becomes greater as the amount of coupler solvent, relative to coupler, is decreased, as noted in U.S. Pat. No. 5,112,729.
Phenolic cyan dye forming couplers are well-known in the art, and are known to be very prone to crystallization. Due to their crystalline nature, these couplers are often dispersed as mixtures of two or more couplers to avoid crystallization problems, as described in U.S. Pat. No. 4,885,234 and EP 434,028. However, use of these methods requires the synthesis of an additional photographic coupler which results in an increase in manufacturing cost. It is also well-known, as mentioned in U.S. Pat. No. 2,801,170, that coupler solvent reduction can result in reduced coupler reactivity. This reference also teaches the concept of combining a solvent-free coupler dispersion with a dispersion of a high boiling coupler solvent alone to produce satisfactory dye density. In order to achieve adequate coupler reactivity phenolic cyan dye-forming couplers are commonly dispersed with high-boiling organic solvents as described in U.S. Pat. No. 4,333,999, U.S. Pat. No. 4,609,619, EP 097,042, EP 102,839, EP 389,817, DE 3,624,777, DE 3,700,570, and DE 3,936,300.
U.S. Pat. No. 5,112,729 describes a photographic material containing a cyan naptholic coupler and a high-boiling solvent present in a weight ratio with respect to the coupler in the layer of not more than 0.3. Methods of increasing the dye yield of oil-free cyan coupler dispersions are disclosed in Research Disclosure 14532 (May, 1976). The specific cyan couplers described in these references fall outside the scope of the present invention. Crystallization of phenolic cyan couplers is also discussed in EP 361,924.
It would be desirable to provide a coupler dispersion which contains no permanent solvent which exhibits good stability toward crystallization and provides adequate coupler reactivity when coated in a silver halide light-sensitive photographic material.